1. Field of the Invention
The present invention relates to an antenna apparatus for use in a millimeter-wave radar system on automobiles.
2. Description of the Prior Art
Millimeter-wave radar systems have been developed for incorporation in automobile collision warning systems. The millimeter-wave radar system on an automobile scans a surrounding area with a radio wave beam to detect the distance up to a reflective object (hereinafter also referred to as a "target") such as a preceding automobile and the direction (hereinafter also referred to as "bearing") of the target as viewed from the automobile that is equipped with the millimeter-wave radar system. Furthermore, the radio wave beam scanning is carried out for not only detecting the distance and bearing of the target, but also expanding and changing a range in which to detect the target as the automobile is turning. Consequently, the millimeter-wave radar system effects the radio wave beam scanning to serve multiple purposes.
Basically, electronic and mechanical radio wave beam scanning schemes have been known in the art. The electronic radio wave beam scanning systems include a system which has a plurality of antennas that successively radiate radio wave beams, respectively, and a system known as a phased array comprising a plurality of radiating elements for generating a radiation pattern whose direction is changed by changing the phases of currents supplied to the radiating elements.
One mechanical radio wave beam scanning system has a rotatable transmitter/receiver unit including an antenna as disclosed in Japanese laid-open patent publication No. 6-273512. Japanese laid-open patent publication No. 7-154126 reveals another mechanical radio wave beam scanning system which has a main reflecting mirror for converging and radiating a radio wave beam that has been scanned by another reflecting mirror.
The electronic radio wave beam scanning systems are relatively large in size and expensive to manufacture because they require a plurality of antennas or elements.
Some of the mechanical radio wave beam scanning systems have a large moving assembly with a large moment of inertia. Therefore, they impose a large burden on the mechanism which actuates the large moving assembly, making it difficult to scan the radio wave beam at a high speed. In order to reduce the size of the moving assembly, it has been proposed to install a primary radiator disposed in confronting relation to the main reflecting mirror and actuate a small-size reflector positioned between primary radiator and the main reflecting mirror. According to the proposed arrangement, the primary radiator and an actuator mechanism for actuating the reflector are positioned in front of the main reflecting mirror. Consequently, the actuator mechanism tends to block the radio wave beam radiated from the main scanning mirror, adversely affecting the beam radiation characteristics of the antenna.